1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor layer made of a finely patterned thin film, a method of manufacturing such a semiconductor device, a display device using such a semiconductor device, and a method of manufacturing such a display device.
2. Description of the Related Art
Thin-film transistors (TFTs) are widely used as pixel transistors in electronic circuits, in particular, in active-matrix-driven flat display units. Recently, the use of an organic material as a semiconductor layer used for such a semiconductor device having a small thickness has attracted attention. In the case of the semiconductor device using the organic material as the semiconductor layer, such a layer can be formed at a temperature lower than that of a semiconductor device using an inorganic material as a semiconductor layer. Therefore, such organic thin-film transistors are advantageous in that thin-film transistors having a large area can be produced, and organic thin-film transistors can be formed on a flexible substrate having poor heat resistance, such as a plastic substrate. Organic thin-film transistors are promising from the viewpoint of realization of multifunctional devices and reduction in cost.
For patterning the semiconductor layer made of the organic material, a vacuum deposition process using a printing method, a metal mask, or the like is carried out. In addition to this, other processes are also performed, including a process of selectively removing an organic semiconductor layer portion on the upper side of a resist pattern by a lift-off technique after the formation of an organic material layer from above the resist pattern and a process of pattern-etching a semiconductor layer using a resist pattern as a mask.
However, the vacuum deposition process using the printing method, the mask, or the like has a disadvantage of limited pattern definition. Particularly, in the case of the vapor deposition process using the metal mask, it is difficult to form a pattern on a large-area substrate with good positional accuracy. Furthermore, in the case of the process using the resist pattern, a resist removal solution used for the removal of a resist pattern also damages an organic semiconductor layer and causes problems of an increase in leak current, a decrease in electron mobility, and a change in threshold voltage in the organic semiconductor layer.
Therefore, another process of forming a semiconductor layer has been proposed. This process includes the steps of: forming a partition wall layer (patterned insulating layer) with large steps on a substrate on which a semiconductor layer is to be formed; and depositing the semiconductor layer from above the partition wall layer. Thus, the semiconductor layer is patterned as one constructed of two parts, a lower part and an upper part, which are divided by the above steps. In other words, for example, source/drain electrodes are pattern-formed on a gate insulating film that covers a gate electrode and the partition wall layer is then formed on the source/drain electrodes. Subsequently, a semiconductor layer is deposited from above the partition wall layer. Part of the semiconductor is placed on the upper part of the partition-wall layer. On the other hand, the rest of the semiconductor is divided from such a part and provided as a semiconductor layer that serves as a channel region between the source/drain electrodes under the partition wall layer (see Japanese Unexamined Patent Application Publication No. 2000-269504A, particularly, FIG. 1 and FIG. 6 and the related descriptions thereof; and Stijn De Vusser, et al., “Integrated shadow mask method for patterning small molecule organic semiconductor” (Applied Physics Letters 88, 2006 American Institute of physics, 2006, 103501-1 to 103501-3).
Furthermore, in the case of a liquid display device using a thin-film transistor with the above semiconductor layer as a pixel transistor, pixel electrodes are formed as parts of the same layer as that of source/drain electrodes or as parts of a layer different therefrom on a gate electrode while being connected to the source/drain electrode. After that, in a manner similar to one described above, a partition wall layer is formed on the source/drain electrodes and the pixel electrode and a semiconductor layer is then formed as a channel region between source/drain electrodes. Subsequently, an oriented film is formed from above these layers via an insulating protective film so that the layers can be covered with the oriented film (see Japanese Unexamined Patent Application Publication No. 2000-269504A, particularly, FIG. 1 and FIG. 6).